Taurine can be referred to as 2-aminoethanesulfonic acid and is of the formula H2NCH2CH2SO3H. Taurine is an extremely useful compound because it per se has such pharmacological effects as detoxification effect, fatigue-relieving effect and nourishing and tonifying effect. As a result, taurine finds wide applications as an essential ingredient for human and animal nutrition.
Numerous chemical methods have been known in the prior arts for preparing taurine and related derivatives. The following two methods have been used in industry to manufacture over 50,000 tons of taurine per year, starting from ethylene oxide (the EU process) and monoethanolamine (the MEA process).
According to the EO process, EO is reacted with sodium bisulfite to obtain sodium isethionate, which undergoes ammonolysis to yield sodium taurinate. Neutralization with sulfuric acid results in a mixture of taurine and sodium sulfate.

In the reactions, M stands for cations, which can be ammonium, lithium, sodium, and potassium.
The EO process has been greatly improved in U.S. Pat. No. 8,609,890 by using sulfur dioxide to neutralize the basic solution of taurinate to recover taurine and to regenerate sodium bisulfite.
The disadvantage of the EU process lies in the problematic quality of the product. More specifically, taurine produced via the EO process is a powder, and tends to form a hard cake over a short period of time during storage (in a matter of weeks), possibly due to the presence of unknown impurities. The process involves some serious hazards from the viewpoint of safety since it uses, as raw material, EO, which has extremely strong toxicity and carcinogenicity and is difficult to transport and handle. Moreover, the reaction is carried out at very high temperature (220-280° C.) and pressure (>100 bars).
Starting from MEA, taurine can be prepared by reacting MEA with sulfuric acid to obtain the key intermediate, 2-aminoethyl hydrogen sulfate ester (AES). Subsequent reaction with sodium sulfite yields a complex mixture of taurine, sodium sulfate, sodium sulfite and other impurities.

The distinct advantage of the MEA process over the EO process is the exceptional quality of the final product, for the taurine obtained is in the form of needle crystal and shows excellent stability during transportation and storage. The thus obtained taurine shows no sign of caking even over a long period of storage time. An added advantage is the mild processing conditions for the safe operation of the manufacturing plant.
A detrimental disadvantage of the MEA process is its higher production cost over the EO process. The main cause is its much lower production yield, the most improved in the industry being in the range of 55-63%. On the other hand, the yield in the EO process is about 75%.
The disadvantage of the MEA process is further exasperated by the extended heating over a long period of time in the sulfonation stage, typically 35-40 hours, as the reaction between AES and sodium sulfite is extremely slow.
These conventional EO and MEA processes are further complicated by another problem such as the separation of taurine from sodium sulfate, as both processes yield a product mixture of taurine and inorganic salts. Because the solubility of taurine and sodium sulfate is similar at temperature below 40° C., sodium sulfate, having nearly the same solubility from 40 to 90° C., has to be crystallized and removed at above room temperature to prevent taurine from crystallization. Repeated heating and cooling are required to separate these two components.
Still another problem in these conventional industrial processes is the disposal of waste stream, comprising of residual taurine, inorganic salts, and high content of organic materials.
Attempt has been made to lower the production cost for the MEA process by substituting sodium sulfite with less costly ammonium sulfite. The yield is still insufficient at less than 65% to compete with the EO process on an industrial scale. Moreover, the waste stream, rich in ammonium salts, could not be satisfactorily treated. Satisfactory method to separate taurine from ammonium sulfate remains to be developed.
According to a process disclosed in JPS608254, the reaction solution of AES and ammonium sulfite is first evaporated to dry, then hydrochloric acid is added to dissolve taurine. The insoluble inorganic salt is filtered off and then washed with concentrated hydrochloric acid. Afterwards, the mother liquor is concentrated to dry again, followed by addition of ethanol to crystallize taurine. This complicated process cannot thus be considered as an industrial production process.
According to another process described in CN101100449A, the reaction mixture between AES and ammonium sulfite is directly cooled to crystallize taurine. After filtration, crude taurine is refined by recrystallization from distilled water. The mother liquor, comprised of taurine and ammonium sulfate and excess ammonium sulfite, has to be discarded.
CN102633689 describes a process of reacting AES and ammonium sulfite to produce taurine and to remove the byproduct of ammonium sulfate and excess sulfite with calcium hydroxide. The expensive starting material, ammonium sulfite, and valuable byproduct, ammonium sulfate, are turned into a waste mixture of calcium sulfite and calcium sulfate. In addition, residual calcium sulfate is introduced into the product stream, thus making final purification to a product of pharmaceutical grade more difficult.
To ensure a good economy of the process, ammonium sulfate should be recovered with a low content of taurine, in a form suitable for fertilizer production. Hence, it is important to have available a method for efficient separation among taurine, ammonium sulfate, and ammonium sulfite.
It is an object of the present invention to overcome the disadvantages of the known MEA process to produce taurine in high yield which is very economical and to provide, in addition, advantages, which will become apparent from the following description.
It is another object of the present invention to disclose a process for the recovery of ammonium sulfate from an aqueous solution which contains ammonium sulfate and taurine. The recovered ammonium sulfate contains less than 0.5% by weight of taurine, and is suitable for fertilizer production.
It is a further object of the present invention to provide a cyclic process for treating the bleeding waste solution by esterifying the residual MEA with sulfuric acid to AES, which can be then converted to taurine. This cyclic process ensures that little waste is generated in the overall process.